1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a laser beam printer, a facsimile machine and the like, having a rotatable developing means capable of developing an image formed on an image bearing member such as a photosensitive drum.
2. Related Background Art
FIG. 15 is a schematic example of a multi-color image forming apparatus for forming an image by using an intermediate transfer member. The image forming apparatus is a copying machine or a laser beam printer using an electrophotographic process. Now, a construction and operation of such an image forming apparatus will be briefly explained.
(1) An electrophotographic photosensitive body (referred to as "photosensitive drum" hereinafter) 1 of a rotary drum type as an image bearing member is arranged within an interior of the image forming apparatus (referred to as "within the apparatus" hereinafter), which photosensitive drum 1 is rotated at a predetermined peripheral speed (process speed) in a direction shown by the arrow R1, and an image formation process (described later) is repeatedly effected regarding a surface of the photosensitive drum. PA1 (2) The charging of the photosensitive drum 1, the image exposure L corresponding to a second color component image (for example, a cyan component image), the development with cyan (C) toner by a second developing device (cyan developing device) 42, the transferring of the second cyan toner image onto the intermediate transfer belt 50, and the cleaning of the surface of the photosensitive drum 1 by the cleaning device 14 are effected. PA1 (3) The charging of the photosensitive drum 1, the image exposure L corresponding to a third color component image (for example, an yellow component image), the development with yellow (Y) toner by means of a third developing device (yellow developing device) 43, the transferring of the third yellow toner image onto the intermediate transfer belt 50, and the cleaning of the surface of the photosensitive drum 1 by the cleaning device 14 are effected. PA1 (4) The charging of the photosensitive drum 1, the image exposure L corresponding to a fourth color component image (for example, a black component image), the development with black (BK) toner by a fourth developing device (black developing device) 44, the transferring of the fourth black toner image onto the intermediate transfer belt 50, and the cleaning of the surface of the photosensitive drum 1 by the cleaning device 14 are effected.
When the photosensitive drum 1 is rotated in the direction R1, it is charged with a predetermined polarity and a predetermined surface potential by a charge means 2 such as a corona charger, and then an electrostatic latent image corresponding to a first color image component (for example, a magenta color image component) of a desired color image is formed on the photosensitive drum by effecting image exposure L by an exposure device 3 (image focusing exposure optical system using color decomposition of an original color image, or a scan exposure optical system using a laser scanner for outputting a laser beam modulated in response to an electric time-sequence digital pixel signal of image information).
Thereafter, the electrostatic latent image is developed with a first color (for example, a magenta (M) color toner (coloring charge particles)) by a first developing device (for example, a magenta developing device) 41 of a rotatable developing means 4. An endless intermediate transfer belt (intermediate transfer member) 50 is arranged below and slightly rightwardly of the photosensitive drum 1. The intermediate transfer belt 50 is mounted around and tensioned by one conductive roller 6 and three turn rollers 7a, 7b and 7c. The conductive roller 6 serves to urge the intermediate transfer belt 50 against the photosensitive drum 1 with a predetermined urging force.
The intermediate transfer belt 50 is rotated in a direction shown by the arrow R5 at a peripheral speed that is the same as that of the photosensitive drum 1. A transfer bias having a polarity (plus) opposite to a charging polarity (minus in the illustrated example) of the toner of a toner image formed on the photosensitive drum 1 is applied to the conductive roller 6 by a first bias power source 61. The intermediate transfer belt 50 is formed from a dielectric sheet made of polyester, polyethylene or the like, or a composite dielectric film comprised of middle-resistance rubber a back surface (inner surface) of which is backed by conductive body. The first color or magenta color toner image formed on the photosensitive drum 1 is transferred onto an outer surface of the intermediate transfer belt 50 at a transfer station by an electric field generated by the application of the transfer bias to the conductive roller 6.
On the other hand, after the first magenta color toner image is transferred to the intermediate transfer belt 50, the photosensitive drum 1 is cleaned by a cleaning device 14.
By successively performing the above image forming and transferring processes (1)-(4), the four toner images (magenta, cyan, yellow and black toner images) are successively transferred onto the outer surface of the intermediate transfer belt 50 in a superimposed fashion, thereby forming a composite color toner image (mirror image) corresponding to a desired color image.
On the other hand, a transfer material (paper sheet) P is separated and supplied one by one from a sheet supply cassette 9 by a sheet supply roller 10, and the separated transfer sheet P is sent to the transfer station defined by a transfer device (corona charger) 7 and the turn roller 7a through a pair of regist rollers 11 and a transfer guide 12 at a predetermined timing. Further, when the toner image is transferred onto the transfer sheet P supplied at the predetermined timing, a transfer bias having a polarity (plus) opposite to a toner charging polarity (minus in the illustrated case) is applied to the transfer device 7 by a third bias power source 71.
By repeating a series of the above-mentioned image forming processes, the color toner images are successively transferred onto the intermediate transfer belt 50, and the transferred color toner images are transferred onto the transfer sheet P supplied to the transfer station. The transfer sheet P to which the toner images were transferred at the transfer station is sent, through a convey guide 13, to a fixing device 15, where the toner images are fused and mixed between a fixing roller 16 and a pressure roller 17 within heat and pressure to form a permanent color image on the transfer sheet. Then, the transfer sheet is discharged out of the image forming apparatus as a color copy.
On the other hand, after the transferring operation, the intermediate transfer belt 50 is cleaned by a belt cleaning device 8. The belt cleaning device 8 is a cleaning device for the intermediate transfer belt 50 and is normally inoperative to the intermediate transfer belt 50. However, after the toner images were transferred onto the transfer sheet P, the belt cleaning device 8 is abutted against the outer surface of the intermediate transfer belt 50, thereby cleaning the outer surface of the intermediate transfer belt 50.
However, in the above-mentioned example, since the rotatable developing means is used as a developing means, the toner is apt to be scattered by the rotation of the developing means, thereby smudging the exposure device 3 and/or the convey guide 13 for the transfer sheet, with the result that the exposure of the photosensitive drum (image bearing member) 1 becomes insufficient and causes poor image formation and/or the transfer sheet is smudged by toner during the conveyance of the transfer sheet.
Incidentally, the Japanese Patent Laid-open No. 5-241420 discloses a technique in which a black developing device is arranged above a rotatable developing means. However, in this case, an exposure device may be smudged by toner.